The field of the present invention is measurement of engine performance.
Internal combustion engines produce power by using the oxygen in air to burn fuel to produce heat, which is then converted into mechanical power. The primary factor controlling the maximum available power level of any given internal combustion engine is the amount of air (and therefore oxygen) that is available to burn fuel. Since burning fuel is a chemical reaction, there are appropriate ratios of the number of oxygen molecules needed to burn the fuel molecules present with optimum combustion efficiency and minimum harmful exhaust by products.
The volume of air available is not alone determinative of how many oxygen molecules will be present. How closely the molecules are packed into this volume, density, must also be measured. Density is measured as mass of air divided by the volume occupied by that mass of air. For “ideal gasses” like air, this relationship is defined by the Ideal Gas Law as PV=nRT. In this equation, P is pressure, V is volume, n is the number of molecules, R is a constant depending on the units used, and T is temperature. This can be algebraically rearranged to nR/V=P/T. This shows that to increase the number of molecules of air (n) in a fixed volume (V), you must either increase the pressure (P), or decrease temperature (T), or do both. This relationship additionally explains why the power output of internal combustion engines, particularly those which are normally aspirated, are affected by the prevailing ambient air conditions of temperature and barometric pressure, as this determines the maximum available intake air density, and therefore the maximum power.
Another factor impacting the number of oxygen molecules is a variation in the constituent mixture of air. Humidity is the only significant natural variable in the constituent mixture as applied to conventionally employed engines. By contributing water molecules to the mixture defining ambient air, humidity has an impact on the percentage of the mix which is oxygen. The influence of humidity is not reflected in the Ideal Gas Law but, when the water remains in a gaseous state, humid air mixture also can be considered to conform to the ideal gas model. Albeit impacting on engine performance, humidity is usually not controlled or controllable in internal combustion engines.
The intake air density can be intentionally changed to control the power output. A spark ignition (SI or Otto cycle) internal combustion engine uses, changes in intake air density as the primary means for controlling power output levels. This method is somewhat unique to SI engines because they must operate within a relatively narrow range of allowable air/fuel ratios. They are unable to reduce power output, for example, by simply restricting the fuel input while running with excess air as can a compression ignition (CI or Diesel cycle) engine. SI engines typically limit power by lowering the engine intake air density below ambient pressure with a throttle. This is a valve located in the intake air tract that generates an adjustable pressure drop in the flow of intake air as it is closed, thereby controllably decreasing the density and, therefore, the power level.
At maximum power, normally aspirated SI and CI engines face the same limitation: they cannot burn more fuel than the amount of oxygen available in air at existing atmospheric pressures and temperatures. Modern high performance engines frequently employ devices to increase the available engine power beyond this limitation by compressing the intake air to increase its density. This is commonly accomplished with devices called turbochargers or superchargers.
Unfortunately, compressing air-causes an increase in the air temperature. Looking again at the Ideal Gas Law: an increase in temperature adversely affects density. However, the temperature of the compressed air is now above ambient, creating an opportunity for easily transferring heat and thereby further increasing density and the obtainable power. Temperature reduction can be accomplished with heat exchange devices commonly called charge air coolers and intercoolers
Thus, mechanisms are available for modifying the limitations on SI and CI engines. Resulting density is a principal factor in the effectiveness of such modifications.
Various devices are known to monitor and control certain engine functions. Devices to indicate power level for engines using turbochargers or superchargers are known which measure the intake manifold pressure. These are commonly called “boost” gauges as they measure the additional pressure above atmospheric provided by the turbocharger or supercharger to “boost” the power output. Some versions also indicate pressure below atmospheric as a manifold vacuum gauge for an SI engine, or both above and below sea level ambient as in an aircraft manifold “absolute pressure” gauge. This gives the operator of an SI engine at full throttle or a CI engine at the maximum fuel setting a rough indication of the effectiveness of the boost device and of its relative impact on power produced.
Devices to measure manifold temperatures are also available. These can measure the effectiveness of an intercooler or reflect adverse engine conditions. They do not provide any measure of engine performance.
The equivalent of density measurements are available for use for internal engine control. Many modern computer controlled SI engines measure both intake manifold air temperature and pressure for use in determining the amount of fuel that is required to achieve the optimum air/fuel ratio. They do not, however, make this information available to the operator. Density measurements are unknown for application as a means for indicating component and system performance or power levels to the operator of the internal combustion engine.
Some CI systems indicate engine “load” based on the quantity of fuel supplied. They cannot sense when ambient conditions, or a system malfunction or degradation is limiting available air density, impacting power.
Personal Digital Assistants (PDA) or similar small hand held computers are used with engines to compute or display engine or vehicle data. Such devices principally have been diagnostic tools to measure various engine parameters and signals which do not provide an indication of overall performance.